Absorbent articles comprising fluid acquisition zones with superabsorbent polymers

ABSTRACT

The present invention relates to absorbent articles comprising superabsorbent polymer particles especially suitable for use in the fluid acquisition zone of the absorbent core. The superabsorbent polymer particles are coated with cationic polymers having 1 to 25 mol/kg, referring to the total weight of the cationic polymers, of cationic groups, which can be protonated. The cationic polymers are not substantially covalently bound to the superabsorbent polymer particles.

This application claims priority to European Application No. 03021716.0filed Sep. 25, 2003.

FIELD OF THE INVENTION

The present invention relates to absorbent articles, which are intendedto receive and retain bodily discharges such as urine. Such articlesinclude disposable hygiene articles such as baby diapers, trainingpants, adult incontinence articles, feminine care articles and the like.To provide absorbent articles with thinner and dryer absorbent coresonly became possible with the development of new highly absorbent gelmaterials able to acquire and store liquids. A second aspect is theability to maintain the comfort and performance of such high superabsorbent polymer concentration articles during the full usage cycle ofthe article, from dry to fully loaded, especially by improving theability of the cores to withstand the forces experienced by sucharticles during use. This ability to remain intact is also oftenreferred to as ‘wet integrity’ of the core.

The development and improvement of highly absorbent gel materials so farmainly focused on applications of these materials in the fluid storagezone of the absorbent core. In contrast thereto, the highly absorbentgel materials of the present invention are especially suitable for usein the fluid acquisition zone of the absorbent core.

BACKGROUND OF THE INVENTION

Absorbent articles for receiving and retaining bodily discharges (e.g.,urine or faeces) such as disposable diapers, training pants, and adultincontinence articles are well known in the art, and significant efforthas been spent against improving their performance. Such improvementsgenerally aim at addressing the primary function of such articles,namely retaining body fluids, but also at minimizing the negativesassociated with wearing such articles by increasing the comfort of thewearer.

Many improvements are related to the “absorbent core” of the absorbentarticle. In the absorbent core the waste material is acquired by thearticle (picked up), then conducted away from the location ofacquisition (distributed), and then stored (retained).

It is well established that reducing the thickness of the article byreducing the thickness of the absorbent core, helps to improve comfort.This, however, was always a question of balance between liquid handlingperformance and thickness. Also a substantial amount of cushioning wasconsidered necessary for comfortable diapers. Finally the skilled personconsidered it impossible to reduce or even remove the fibrous materialcommonly applied in absorbent cores to a point where the modernparticulate super absorbent materials would take over the biggest partor even all of the liquid acquisition and distribution functionalitiespreviously provided by fibrous matrixes. Even if there were structureswhich could possibly provide all such beneficial aspects when dry, itwas completely inconceivable that this could be built into an absorbentcore such that the liquid handling and comfort performance would bemaintained even after the first gushes of liquid had been absorbed.

The development of absorbent cores of particular thinness has beneficialaspects, which make such a development the subject of substantialcommercial interest. For example, thinner diapers are not just lessbulky to wear, conform better to the body and fit better under clothing,they are also more compact in the package, making the diapers easier forthe consumer to carry and store. Compactness in packaging also resultsin reduced distribution costs for the manufacturer and distributor,including less shelf space required in the store per diaper unit.

As indicated, the ability to provide thinner absorbent articles such asdiapers has been dependent on the ability to develop relatively thinabsorbent cores that can acquire and store large quantities ofdischarged body fluids, in particular urine. In this regard, the use ofabsorbent polymers often referred to as “hydrogels,” “super absorbents”or “hydrocolloid” material has been particularly important. See, forexample, U.S. Pat. No. 3,699,103 and U.S. Pat. No. 3,770,731, thatdisclose the use of such absorbent polymers (hereafter referred to asany of the following: hydrogel forming absorbent polymers, superabsorbents, super absorbent polymers or SAPs, absorbent gel materials orAGMs). Indeed, the development of thinner diapers has been the directconsequence of thinner absorbent cores that take advantage of theability of these SAPs to absorb large quantities of discharged bodyfluids, typically when used in combination with a fibrous matrix. See,for example, U.S. Pat. No. 4,673,402 and U.S. Pat. No. 4,935,022, thatdisclose dual-layer core structures comprising a fibrous matrix and SAPsuseful in fashioning thin, compact, non-bulky diapers.

SAPs are often made by initially polymerizing unsaturated carboxylicacids or derivatives thereof, such as acrylic acid, alkali metal (e.g.,sodium and/or potassium) or ammonium salts of acrylic acid, alkylacrylates, and the like. These polymers are rendered water-insoluble,yet water-swellable, by slightly and homogeneously cross-linking thecarboxyl group-containing polymer chains with conventional di- orpoly-functional monomer materials, such asN,N′-methylene-bisacryl-amide, trimethylol-propane-triacrylate ortriallyl-amine. These slightly cross-linked absorbent polymers stillcomprise a multiplicity of anionic (charged) carboxyl groups attached tothe polymer backbone. It is these charged carboxyl groups that enablethe polymer to absorb body fluids as the result of osmotic forces, thusforming hydrogels.

It is often desirable to increase the stiffness of SAP particles.Typically, this is done by surface cross-linking, wherein thecarboxyl-groups exposed on the surface of the SAP particles areadditionally cross-linked to each other. Other methods to increase thestiffness comprise coating the SAP particles. Such coatings are knownfor example from WO 97/12575, which discloses absorbent materialscomprising absorbent gelling particles and a polycationic polymercovalently bonded to the absorbent gelling particles. European patentapplication EP 493 011 A2 refers to an absorbent matter comprising awater-absorbent resin particle having an acidic group on the surface, acellulose fiber and a cationic high-molecular compound having aweight-average molecular weight of 2,000 or more. Further, WO 03/043670discloses superabsorbent particles with a shell comprising cationicpolymer cross-linked by the addition of cross-linker and adhered to thehydrogel-forming polymer by applying a coating solution containing botha cationic polymer and cross-linker.

However, the development and improvement of SAPs has so far mainlyfocused on use of the SAPs for final storage of liquid in the absorbentcore. Consequently, it would be desirable to have SAPs, which areespecially suitable for fluid acquisition and distribution in the fluidacquisition zone of the absorbent core. By replacing the fibers used inthe fluid acquisition zone of prior art absorbent cores with SAPs, itwould be possible to further reduce the bulk and thickness of theabsorbent core.

A problem in developing SAPs for use in the fluid acquisition zone isthat the demands on the physical and chemical properties of these SAPsdiffer considerably from the requirements for use in the fluid storagezone. For example, it is desirable that SAPs in the fluid acquisitionzone are able to quickly acquire via capillary pressure and temporarilyhold fluids in voids between the SAP particles, especially in “gush”situations. The focus is not primarily on SAPs with high capacity, butit is important that a hydrogel-bed formed from SAPs has high porosityand permeability in order to be able to provide enough intersticesbetween the swollen SAPs.

It would also be desirable to have SAP particles for use in absorbentcores, especially in the fluid acquisition zones, which have a high FreeSwelling Rate (FSR). The FSR is related to the surface area of the SAPparticles, and thus to their particle size, particle shape andmorphology of the particles (e.g., a porous SAP particle versus anon-porous SAP particle). SAP particles having a high surface areatypically also have a high FSR, which means they are able to quicklyabsorb liquids. The ability to quickly absorb liquids is especiallyimportant in the fluid acquisition zone of absorbent cores to createsufficient void volume. It is especially desirable to have SAP particlesfor use in the fluid acquisition zone, which are highly saturatedalready after the fist gush of liquid.

Moreover, it would be desirable to be able to provide an absorbent corecomprising an acquisition zone having a relatively high concentration ofSAP particles with relatively high porosities, and high permeability,and in a matrix that provides sufficient wet integrity such that itscapability for acquiring and transporting fluids is not substantiallyreduced or minimized, even when subjected to normal use forces.

It would also be highly desirable to be able to use SAPs in theseabsorbent cores, especially in the fluid acquisition zones, that, whenswollen by body fluids, continue to have a good wet integrity and highporosity such that the void volume per unit weight of absorbent polymerrelatively high and preferably is close to that of an air-laid fibrousweb, such as have been used in fluid acquisition zones of prior artabsorbent articles.

Hence it is an object of the present invention to provide absorbentarticles having an improved fit especially by reducing their thicknessbut also when being loaded, together with good fluid handlingperformance, especially by using materials having particularly suitablefluid distribution properties when dry and during progressive saturationwith liquids.

It is a further object of the invention to achieve this by using superabsorbent polymers particles.

SUMMARY OF THE INVENTION

In one embodiment, the present invention relates to absorbent articlescomprising a substantially liquid pervious topsheet, a substantiallyliquid impervious backsheet and an absorbent core between the topsheetand the backsheet. The topsheet and the backsheet are at least partiallyjoined together. The absorbent core comprises at least one fluidacquisition zone wherein the fluid acquisition zone comprisessuperabsorbent polymer particles, characterized in that:

-   a) the superabsorbent polymer particles further are surface coated    with cationic polymers, wherein the cationic polymers have 1 to 25    mol/kg, referring to the total weight of the cationic polymers, of    cationic groups, which can be protonated, and wherein the cationic    polymers are not substantially covalently bound to said    superabsorbent polymer particles, and-   b) the superabsorbent polymer particles have a Cylinder Centrifuge    Retention Capacity (CCRC) of less than 25 g/g.

An alternative embodiment of the present invention relates to absorbentarticles comprising a substantially liquid pervious topsheet, asubstantially liquid impervious backsheet and an absorbent core betweenthe topsheet and the backsheet. The topsheet and the backsheet are atleast partially joined together. The absorbent core comprises at leastone fluid acquisition zone wherein the fluid acquisition zone comprisessuperabsorbent polymer particles, characterized in that:

-   a) the superabsorbent polymer particles further are surface coated    with cationic polymers, wherein the cationic polymers have 1 to 25    mol/kg, referring to the total weight of the cationic polymers, of    cationic groups, which can be protonated, and wherein the cationic    polymers are not substantially covalently bound to said    superabsorbent polymer particles, and-   b) the fluid acquisition zone comprises the superabsorbent polymer    particles in a concentration of at least 50% by weight of the fluid    acquisition zone.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out anddistinctly claiming the present invention, it is believed the same willbe better understood by the following drawings taken in conjunction withthe accompanying specification wherein like components are given thesame reference number.

FIG. 1 is a top plan view of a disposable diaper, with the upper layerspartially cut away.

FIG. 2 is a cross-sectional view of the disposable diaper shown in FIG.1

DETAILED DESCRIPTION OF THE INVENTION

As used herein, absorbent article refers to devices that absorb andcontain liquid, and more specifically, refers to devices that are placedagainst or in proximity to the body of the wearer to absorb and containthe various exudates discharged from the body. Absorbent articlesinclude but are not limited to diapers, adult incontinent briefs, diaperholders and liners, sanitary napkins and the like. Preferably, theabsorbent articles of the present invention are disposable absorbentarticles.

Preferred absorbent articles of the present invention are diapers. Asused herein, “diaper” refers to an absorbent article generally worn byinfants and incontinent persons about the lower torso.

“Disposable” is used herein to describe articles that are generally notintended to be laundered or otherwise restored or reused i.e., they areintended to be discarded after a single use and, preferably, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner.

“Disposed” is used to mean that an element(s) is formed (joined andpositioned) in a particular place or position as a unitary structurewith other elements or as a separate element joined to another element.

As used herein, the term “absorbent core” refers to a component of anabsorbent article that is primarily responsible for fluid handlingproperties of the article, including acquiring, transporting,distributing and storing body fluids. As such, the absorbent coretypically does not include the topsheet or backsheet of the absorbentarticle.

The absorbent core of the present invention comprises at least one fluidacquisition zone and at least one fluid storage zone. The fluidacquisition zone and the fluid storage zone may be portions or sectionsof an absorbent core.

The absorbent core may comprise two or more layers, wherein the fluidacquisition zone may comprise at least one layer and the fluid storagezone may comprise at least one layer.

The fluid acquisition zone is directed towards the wearer and the fluidstorage zone is directed towards the garment, while the absorbentarticle is in use.

As used herein, the term “comprising” means that e.g., variouscomponents, zones, layers, steps and the like can be conjointly employedaccording to the present invention. Accordingly, the term “comprising”encompasses the more restrictive terms “made of” and “consisting of,”these latter, more restrictive terms having their standard meaning asunderstood in the art.

The present invention relates to absorbent cores useful in the provisionof absorbent incontinence articles such as baby diapers or adultincontinence articles, which articles preferably comprise a topsheet, abacksheet and an absorbent core sandwiched between the topsheet and thebacksheet.

The absorbent cores of the present invention are especially useful forcollection of bodily liquids such as urine. Such cores comprise superabsorbent polymers (SAP), which are in the form of particles (SAPparticles).

Preferably, the SAP particles are present in the fluid acquisition zoneof the absorbent core in a concentration of at least 50% by weight ofthe fluid acquisition zone, more preferably in a concentration of atleast 60% by weight.

The SAP particles may be of numerous shapes. The term “particles” refersto granules, fibers, flakes, spheres, powders, platelets and othershapes and forms known to persons skilled in the art of SAPs. Forexample, the particles can be in the form of granules or beads, having aparticle size from about 10 μm to about 1000 μm, preferably from about100 μm to about 1000 μm, even more preferably from about 150 μm to about850 μm and most preferably from about 150 μm to about 500 μm. In anotherembodiment, the SAPs can be in the shape of fibers, i.e. elongated,acicular SAP particles. In those embodiments, the SAP fibers have aminor dimension (i.e. diameter of the fiber) of less than about 1 mm,usually less than about 500 μm, and preferably less than 250 μm down to50 μm. The length of the fibers is preferably about 3 mm to about 100mm. The fibers can also be in the form of a long filament that can bewoven.

Preferred SAPs of the present invention are spherical-like particles.According to the present invention and in contrast to fibers,“spherical-like particles” have a longest and a smallest dimension witha particulate ratio of longest to smallest particle dimension in therange of 1-5, where a value of 1 would equate a perfectly sphericalparticle and 5 would allow for some deviation from such a sphericalparticle.

The SAPs useful in the present invention include a variety ofwater-insoluble, but water-swellable polymers capable of absorbing largequantities of fluids. Such polymers materials are generally known in theart and include all those well-known polymers used or deemed useful inthe context of disposable absorbent article technology. Particularly theSAPs disclosed in EP-A-752 892 or those disclosed in a textbook entitled“Modern Super Absorbent Technology” by F. L. Buchholz and A. T. Graham,published by Wiley VCH, New York, 1998 are useful in the context of thepresent invention.

Preferred polymer materials for use in making SAP particles are slightlynetwork cross linked polymers of partially neutralized polyacrylic acidsand starch derivatives thereof. Preferably, the SAP particles comprisefrom 25% to 95% by weight, more preferably from 50% to 80% by weight,neutralized, slightly network cross-linked, polyacrylic acid. Networkcross-linking renders the polymer substantially water-insoluble and, inpart, determines the absorptive capacity and extractable polymer contentcharacteristics of the hydrogel-forming absorbent polymers. Processesfor network cross linking these polymers and typical networkcross-linking agents are described in greater detail in U.S. Pat. No.4,076,663 or references cited supra.

While the SAP is preferably of one type (i.e., homogeneous), mixtures ofpolymers can also be used in the present invention. The SAPs can alsocomprise mixtures with low levels of one or more additives, such as forexample powdered silica, surfactants, glue, binders, and the like.Furthermore, the SAPs can comprise a gradient in particle size or cancomprise a certain range of particle size.

Many of the known SAPs exhibited gel blocking. “Gel blocking” occurswhen particles of the SAP are wetted and the particles swell so as toinhibit fluid transmission to other zones or regions of the absorbentstructure. Wetting of these other regions of the absorbent coretherefore takes place via a very slow diffusion process. In practicalterms, this means acquisition of fluids by the absorbent structure ismuch slower than the rate at which fluids are discharged, especially ingush situations. Leakage from the absorbent article can take place wellbefore the particles of SAP in the absorbent core are even close tobeing fully saturated or before the fluid can diffuse or wick past the“blocking” particles into the rest of the absorbent core. Gel blockingcan be a particularly acute problem if the particles of SAP do not haveadequate gel strength and deform or spread under stress once theparticles swell with absorbed fluid. See U.S. Pat. No. 4,834,735. Theproblem of gel blocking is especially critical in the present invention,because the SAP particles are preferably applied in the fluidacquisition zone. In the fluid acquisition zone, the main task of theSAP particles is not to store liquid as is the case in most prior artuses of SAP particles in absorbent articles but the main task is toascertain that liquid can pass through the fluid acquisition zonesufficiently quickly and to temporarily store liquid, which cannot beabsorbed quick enough by the fluid storage zone.

One commonly applied way to reduce gel blocking is to make the particlesstiffer, which enables the SAP particles to retain their original shapethus creating or maintaining void spaces between the particles. Awell-known method to increase stiffness is to covalently cross-link thecarboxyl groups exposed on the surface of the SAP particles. This methodis commonly referred to as surface cross-linking.

The term “surface” describes the outer-facing boundaries of theparticle. For porous SAP particles, exposed internal surfaces may alsobelong to the surface. The term “surface cross-linked SAP particle”refers to an SAP particle having its molecular chains present in thevicinity of the particle surface cross-linked by a compound referred toas surface cross-linker. The surface cross-linker is applied to thesurface of the particle. In a surface cross-linked SAP particle thelevel of cross-links in the vicinity of the surface of the SAP particleis generally higher than the level of cross-links in the interior of theSAP particle.

Surface cross-linkers known in the prior art are e.g., di- orpolyfunctional agents that are capable of building additionalcross-links between the polymer chains of the SAPs. Surfacecross-linkers include, e.g., di- or polyhydric alcohols, or derivativesthereof, capable of forming di- or polyhydric alcohols. Thecross-linking is based on a reaction between the functional groupscomprised by the polymer, for example, an esterification reactionbetween a carboxyl group (comprised by the polymer) and a hydroxyl group(comprised by the surface cross-linker).

According to the present invention the SAP particles are surface coatedwith cationic polymers. The cationic polymers have 1 to 25 mol/kg,referring to the total weight of the cationic polymers, of cationicgroups, which can be protonated. Furthermore, the cationic polymers arenot substantially covalently bonded to the superabsorbent particles.Preferably, the cationic polymers have 3 to 22 mol/kg of cationicgroups, which can be protonated, more preferably 3-17 mol/kg of cationicgroups, which can be protonated, even more preferably 4-14 mol/kg ofcationic groups, which can be protonated and most preferably 4-12 mol/kgof cationic groups, which can be protonated.

However, preferably the cationic polymers are nitrogen-containingpolymers (N-polymers) having 1 to 25 mol/kg (based on the weight of thenitrogen containing polymer) nitrogen atoms, which can be protonated.More preferably, the nitrogen-containing cationic polymers have 3 to 22mol/kg of nitrogen atoms, which can be protonated, still more preferably3-17 mol/kg of nitrogen atoms, which can be protonated, even morepreferably 4-14 mol/kg of nitrogen atoms, which can be protonated andmost preferably 4-12 mol/kg of nitrogen atoms, which can be protonated.

In a preferred embodiment of the present invention, thenitrogen-containing cationic polymer is polyethyleneimine (PEI).

In another preferred embodiment of the present invention, thenitrogen-containing cationic polymer is a partially hydrolyzed polymer.If a partially hydrolyzed nitrogen-containing cationic polymer is usedfor the surface coating, this polymer is preferably hydrolyzed in therange of 30%-80%, more preferably in the range of 40%-70%, and mostpreferably in the range of 40%-60%. The degree of hydrolyzation has amajor impact on the overall charge of the cationic polymer, because onlythe hydrolyzed groups comprised by the coating polymer are positivelycharged.

A detailed description of partially hydrolyzed or at least hydrolysablepolyvinyl-amides and how to make them is found in DE 31,28,478.Particularly preferred are nitrogen-containing cationic polymersprovided by a polymer made from a homo-polymerization ofN-vinyl-form-amide (PVFA). The PVFA is at least partially hydrolyzed topolyvinyl-amine (PVAm), preferably with a hydrolysation degree of fromabout 30 mol % to about 80 mol %. Solutions of partially hydrolyzedpolyvinyl-form-amides are available commercially, e.g., from BASF-AG,Ludwigshafen, Germany, under the trade names Lupamin™ and Luredur™. In aparticular embodiment of the present invention is the coating is apartially hydrolyzed polymer of N-vinyl-alkyl-amide orN-vinyl-alkyl-imide, more preferably a partially hydrolyzed polymer ofN-vinyl-formamide.

According to the present invention, nitrogen-containing cationicpolymers with primary, secondary and tertiary amines are preferred.

The cationic polymers preferably are added on the SAP particles in anamount of less than 10% by weight of said SAP particles, more preferablybetween 0.05% and 5% and even more preferably between 0.1% and 1.0% byweight of the SAP particles without cationic polymers (uncoated SAPparticles).

Without wishing to be bound by this theory, it is believed that thecationic polymers adhere to the surface of the SAP particles via ionicbonds, Van-der-Waals forces and hydrogen bonds.

Preferably, the cationic polymers of the present invention have aweight-average molecular weight in the range of about 10,000 to about1,000,000, more preferably in the range of about 10,000 to about 800,000and even more preferred in the range of about 50,000 to about 750,000.

Physical Properties of the SAP Particles

1. Permeability

Permeability (also called liquid flow conductivity) is an importantproperty of the coated SAP particles of present invention. Permeabilityof the SAP particles is determined based on a modification of the methodto determine SFC (Saline Flow Conductivity). SFC measures the ability ofa material to transport saline fluids, e.g., the ability of a hydrogellayer formed from the swollen SAP to transport body fluids under usagepressures. The method to determine SFC is described in EP-B-0 752 892,paragraph 224 and following.

For the permeability measurement of hydrogel-layer formed by SAPparticles of the current invention, 0.9% Saline solution (9.00 g NaCl in1 liter deionised water) instead of synthetic urine (known as JaycoSyUrine or Jayco Synthetic Urine, available from Jayco Pharmaceuticals,Company Hill, Pa., USA) is used during the 60 minutes of pre-swellingthe SAP particles and during the liquid flow measurement. In case of SAPthat forms high permeability hydrogel-layer (i.e., fast liquid flowrate), as is the case for the SAP particles of the present invention, itis particularly necessary to increase the thickness of thehydrogel-layer formed by the SAP particles in order to maintain thefluid height of 5.0 cm above the screen attached to the bottom of thecylinder throughout the whole measurement. For this purpose, thestarting sample amount of dry SAP particles need to be controlled basedon the capacity (Cylinder Centrifuge Retention Capacity) of SAPparticles to adjust the height of the swollen hydrogel-layer formed bySAP particles between 10 mm and 20 mm. Confining pressure of 0.3 psi(2.1 kPa) is applied during the pre-swelling and the flow measurement.

-   -   Measure caliper of empty sample holder assembly to the accuracy        of 0.01 mm: h₀.    -   Weigh dry SAP particles according to the appropriate amount        mentioned above to the accuracy of 0.001 g.    -   Transfer complete sample and evenly distribute the SAP particles        on the whole are of the bottom of the sample holder.    -   Measure caliper of empty sample holder assembly and dry SAP        particle to the accuracy of 0.01 mm: h₁.    -   Put the sample holder assembly onto a filter paper (e.g.,        Schleicher & Schuell, No. 596, 90 mm diameter or equivalent),        submerged in a large petri-dish with 0.9% saline solution where        the liquid height is at least 3.0 cm and pre-swell for 60        minutes.    -   After 60 minutes, remove sample holder assembly from        petri-dish/Saline solution and get excess liquid to drip off.    -   Measure caliper of sample holder assembly and swollen SAP        particles to the accuracy of 0.01 mm: h₂.    -   Transfer the sample holder assembly with samples onto the        support screen and let 200 ml of 0.9% saline solution flow        through the sample while adjusting the 5.0 cm hydrostatic head        above the screen attached to the bottom of the cylinder.    -   Start permeability data acquiring for 5 minutes with a data        recording intervals of 10 sec for the fast flow rate        hydrogel-layer.    -   Let drip-off excess fluid for 30 minutes by holding the outer        wall of the sample holder and to avoid hydrogel-layer formed        from the SAP particles contacting any surface to let the liquid        easily drip off to the reservoir.    -   Measure the caliper of sample holder assembly and hydrogel-layer        to the accuracy of 0.01 mm: h₃.        Calculations of permeability values are based on the flow rate        that is determined via the slope of the uptake-vs-time curve.        The average of three determinations is reported.        (Permeability)K={F _(g) ×L _(final) }/{ρ×A×ΔP}        wherein        F_(g) is the flow rate in [g/sec] determined from the        calculation of the slope of the uptake-vs-time curve, i.e.        liquid amount flowing through the swollen hydrogel-layer at each        10 sec time intervals, and        L_(final) is the average value of the initial thickness of the        SAP-layer after 60 minutes of swelling, h₂−h₀ and the gel layer        thickness after flow and after dripping off the excess liquid,        h₃−h₀ in [cm].        L _(final)=[(h ₂ −h ₀)+(h ₃ −h ₀)]/2×10        wherein        ρ is the density of the NaCl solution in [g/cm³] (1.005 g/cm³        for 0.9% saline solution at 20° C.), and        A is the area of the hydrogel layer 28.27 cm², and        ΔP is the hydrostatic pressure 4920 g·cm/s², and        the permeability, K, is in units of [cm³ s/g].

The SAP particles, which are surface coated with cationic polymersaccording to the present invention, preferably have a measuredpermeability of at least 400×10⁻⁷ cm³ sec/g, more preferably more than800×10⁻⁷ cm³ sec/g, even more preferably more than 1000×10⁻⁷ cm³ sec/gand most preferably more than 1200×10⁻⁷ cm³ sec/g.

2. Wet Porosity

An important characteristic of SAP particles, which is preferablyachieved by the SAP particles according to the present invention, is theopenness or porosity of the hydrogel zone or layer formed when thepolymer is swollen in body fluids under a confining pressure. This isreferred to as Wet Porosity.

The openness or porosity of the hydrogel layer is formed when the SAPparticles swell in the presence of body fluids. The Wet Porosity isimportant to determine the ability of the SAP particles to acquire andtransport fluids, especially when the SAP particles are present at highconcentrations in the absorbent structure. Porosity refers to thefractional volume that is not occupied by solid material. For a hydrogellayer formed entirely from a SAP, porosity is the fractional volume ofthe layer that is not occupied by hydrogel. For an absorbent structurecontaining the hydrogel, as well as other components, porosity is thefractional volume (also referred to as void volume) that is not occupiedby the hydrogel, or other solid components (e.g., fibers).

This test determines the porosity of the hydrogel-layer formed byswollen SAP particles in 0.9% saline solution under a confining pressureof 0.3 psi (2.1 kPa). The objective of this test is to assess theability of the hydrogel-layer formed from SAP particles to remain porouswhen the SAP particles are present in high concentration in an absorbentlayer and exposed to usage mechanical pressure. Wet porosity is thefractional volume of the layer that is not occupied by swollen SAPparticles. See J. M. Coulson et al. Chemical Engineering Vol. 2, 3^(rd)Edition, Pergamon Press, 1978, P 126.

Wet porosity of the current invention is measured together with thepermeability measurement using the same sample and the sample holderassembly (piston/cylinder apparatus) and additional measurements. Inthis way, the porosity data can be derived in addition to the closelyrelated permeability values from the same experimental set-up.

-   -   Weigh empty sample holder assembly to the accuracy of 0.01 g        after the measurement of caliper of empty sample holder (see        permeability measurement): w₀.    -   After the drip-off of excess fluid for 30 minutes (as in        permeability), weigh sample holder assembly and swollen SAP with        fluid in interstitials to the accuracy of 0.01 g: w₁.    -   Dry out the fluid in interstitials by placing the assembly of        SAP particles and sample holder on paper hand towels (Metsa        Tissue, Katrin®, C-fold2) 16 hours (+/−1 or 2 hrs).    -   Measure caliper of sample holder assembly and SAP particles        after interstitials are dried to the accuracy of 0.01 mm: h₄.    -   Weigh sample holder assembly and SAP particles with dried        interstitials to the accuracy of 0.01 g:w₂.

The final wet porosity of swollen SAP particles, n, can be obtained asfollows:(Wet Porosity)n=V _(void) /V _(tot)wherein V_(void) is obtained by the weight of liquid in interstitials ofthe hydrogel-layer obtained from the weight difference between beforeand after the dry out process with an addition to the weight correctionof 0.7 g as below.V _(void)=(w ₁ −w ₀)−(w ₂ −w ₀)−0.7 (weight correction as fluid clingsto sample holder)/ρwhereinρ is the density of the NaCl solution in g/cm³ (1.005 g/cm³ for 0.9%saline solution at 20° C.), andV_(tot) is obtained by wet sample caliper and the area of the hydrogellayer A formed from the SAP particles.V _(tot)=(h ₃ −h ₀/10)×AThe values for h₀, h₃ and A, are those values, which have been measuredin permeability measurement.

In general, the Wet Porosity takes a value between 0 and 1. PreferredSAP particles useful in the present invention have Wet Porosity valuesof at least 0.20, more preferably at least 0.23, even more preferably atleast 0.25. However the porosity remains an important aspect over thewhole usage range of pressures experienced by absorbent cores, i.e.starting from no pressure to pressures such as 10 000 Pa.

3. Cylinder Centrifuge Retention Capacity (CCRC)

This test serves to measure the saline-water-solution retention capacityof the SAP particles used herein, when the SAP particles are submittedto centrifuge forces (and it is an indication of the maintenance of theabsorption capacity of the SAP particles in use, when also variousforces are applied to the material).

First, a saline-water solution is prepared as follows: 18.00 g of sodiumchloride is weighed and added into a two liter volumetric flask, whichis then filled to volume with 2 liter deionised water under stirringuntil all sodium chloride is dissolved.

A pan with a minimum 5 cm depth, and large enough to hold fourcentrifuge cylinders is filled with part of the saline solution, suchthat up to a level of 40 mm (±3 mm).

Each SAP particles sample is tested in a separate cylinder and eachcylinder to be used is thus weighed before any sample is placed in it,with an accuracy of 0.01 g. The cylinders have a very fine mesh on thebottom, to allow fluid to leave the cylinder.

For each measurement, a duplicate test is done at the same time; so twosamples are always prepared as follows:

1.00 g of the SAP particles, which are to be tested, is weighed, with anaccuracy of 0.005 g (this is the ‘sample’), and then the sample istransferred to an empty, weighed cylinder. (This is repeated for thereplica.)

Directly after transferring the sample to a cylinder, the filledcylinder is placed into the pan with the saline solution (Cylindersshould not be placed against each other or against the wall of thepan.).

After 15 min (±30 s), the cylinder is removed from the pan, and thesaline solution is allowed to drain off the cylinder; then, the cylinderis re-placed in the pan for another 45 min. After the total of 60minutes immersion time, the cylinder is taken from the solution andexcess water is allowed to run off the cylinder and then, the cylinderwith the sample is placed in the cylinder stands inside a centrifuge,such that the two replicate samples are in opposite positions.

The centrifuge used may be any centrifuge equipped to fit the cylinderand cylinder stand into a centrifuge cup that catches the emergingliquid from the cylinder and capable of delivering a centrifugalacceleration of 250 g (±5 g) applied to a mass placed on the bottom ofthe cylinder stand (e.g., 1300 rpm for a internal diameter of 264 mm). Asuitable centrifuge is Heraeus Megafuge 1.0 VWR # 5211560. Thecentrifuge is set to obtain a 250 g centrifugal acceleration. For aHeraeus Megafuge 1.0, with a rotor diameter of 264 mm, the setting ofthe centrifuge is 1300 rpm.

The samples are centrifuged for 3 minutes (±10 s).

The cylinders are removed from the centrifuge and weighed to the nearest0.01 g.

For each sample (i), the cylinder centrifuge retention capacity W_(i),expressed as grams of saline-water-solution absorbed per gram of SAPparticles is calculated as follows:

$w_{i} = {\frac{m_{CS} - ( {m_{Cb} + m_{S}} )}{m_{S}}\lbrack \frac{g}{g} \rbrack}$wherein:m_(CS): is the mass of the cylinder with sample after centrifugation [g]m_(Cb): is the mass of the dry cylinder without sample [g]m_(S): is the mass of the sample without saline solution [g]Then, the average of the two W_(i) values for the sample and its replicais calculated (to the nearest 0.01 g/g) and this is the CCRC as referredto herein.

Preferably the CCRC of the SAP particles of the present invention isless than 25 g/g, more preferably less than 20 g/g, still morepreferably less than 19.5 g/g and most preferably less than 17 g/g.Furthermore, the CCRC of the SAP particles of the present invention ismore than 5 g/g, more preferably more than 6 g/g.

4. Free Swell Rate (FSR)

This method serves to determine the swell rate of the SAP particles usedherein in a 0.9% saline solution, without stirring or confiningpressure. The amount of time taken to absorb a certain amount of fluidis recorded and this is reported in gram of fluid (0.9% saline) absorbedper gram of SAP particles per second, e.g., g/g/sec.

The saline solution is prepared by adding 9.0 gram of NaCl into 1000 mldistilled, deionized water, and this is stirred until all NaCl isdissolved.

The sample amount calculated (see the below equation) is weighed (to anaccuracy of 0.0001 g) and placed evenly over the bottom of a 25 mlbeaker; then 20 g of the saline solution (also at 23° C.) is addedquickly to the beaker with the sample and the timer is started. Exactweight of saline solution added is determined to the accuracy of 0.01g:w_(liq)w _(dry)=dry weight[g]=20 [g]/(0.75×CCRC[g/g])

When the last part of the undisturbed fluid surface meets the swellingsample, e.g., judged by the light reflection from the fluid surface, thetime t_(s) is recorded.

The test is repeated twice, to obtain 3 values.

The Free Swell Rate is then calculated per sample and this can beaveraged to obtain the Free Swell Rate, as referred herein.FSR=w _(liq)/(w _(dry) ×t _(s))

Preferably, the FSR of the SAP particles of the present invention is atleast 0.1 g/g/sec, more preferably at least 0.3 g/g/sec, even morepreferably at least 0.5 g/g/sec and most preferably at least 0.7g/g/sec.

5. Acquisition Test

The fluid acquisition test provides a means for introducing fluid intoan absorbent article that simulates in-use conditions. The article willbe loaded with a 75 ml/gush of 0.9% Saline solution at a rate of 15 mls⁻¹ using a pump (Model 7520-00 Cole Parmer Instruments Co., Chicago,USA). The time to absorb saline solution is recorded by a timer. Thegush is repeated every 5 minutes at precisely 5 minutes gush intervalsuntil 4 gushes.

The test sample, which comprises a core and includes a topsheet and abacksheet, is arranged to lie flat on a foam platform within a perspexbox (see the detail assembly of test apparatus in European Patent EP 0631 768 B1 or U.S. Pat. No. 6,083,210). The core has a ultimate storagecapacity of about 300 ml to about 400 ml. If products with significantdifferent capacities should be evaluated (such as can be envisaged foradult incontinence products), the settings in particular of the fluidvolume per gush should be adjusted appropriately to about 20% of thetotal article design capacity, and the deviation from the standard testprotocol should be recorded.

The outer surface of the backsheet is facing the foam platform. APerspex plate having a 5 cm diameter opening substantially in its middleis placed on top of the sample. The sample is oriented such that thetopsheet is directly below the opening of the perspex plate. The openingin the plate (=loading point for the saline solution) is placed about 10cm from the front edge of the complete core and about in the halfwaybetween the lateral sides of the core. Saline solution is introduced tothe sample through the cylinder fitted and glued into the opening.Electrodes are about 1 mm to 2 mm above the surface of the absorbentstructure and also connected to the timer. Loads are placed on top ofthe plate to simulate, for example a baby's weight. A weight of 9 kg isplaced on top of the plate with an area of 744.6 cm² (51 cm×14.6 cm).

As saline solution is introduced into the cylinder. It builds up on topof the absorbent structure thereby completing an electrical circuitbetween the electrodes. This starts the timer. The timer is stopped andrecorded when the absorbent structure has absorbed the gush, and theelectrical contact between the electrodes is broken.

Acquisition rate is defined as the gush volume absorbed (ml) per unittime (s). The acquisition rate is calculated for each gush introducedinto the sample.

Preferred acquisition rates for the 4^(th) gush are at least 0.7 ml/s,more preferably at least 0.8 ml/s.

6. Staining of SAP Particles with Toluidine Blue in 0.9% NaCl

With the following method, it is possible to determine, if SAP particlescoated with cationic polymers have been surface cross-linked prior toapplying the cationic polymer coating on the SAP particles.

Staining Solution (20 ppm Toluidine Blue O in 0.9% NaCl):

20 mg Toluidine Blue O [CAS: 540-23-8] are dissolved in 250 ml 0.9%(w/w) NaCl solution. The mixture is placed into an ultrasonic bath for 1hour, filtered through a paper filter, and filled up to 1000 ml with0.9% NaCl solution.

Staining Procedure:

A sample of 30-50 mg of SAP particles coated with cationic polymers isplaced into a 40 ml screw cap glass vial, and 30 ml of the abovestaining solution are added. The vial is closed, and the SAP particlesare allowed to swell and equilibrate for 18 hours at 20-25° C. duringgentle agitation (e.g., gentle swirling or slow rolling of the vial on aroller mill).

For microscopy assessment, the swollen, stained samples of SAP particlesare transferred into white porcelain dishes, and covered with thesolution in which they were prepared (or placed into 1 cm glass orquartz cuvettes with a stopper in contact with this solution).

A stereomiscroscope (e.g., Olympus Stereomicroscope SZH10 (7-70×),equipped with a circular illumination (e.g., Intralux UX 6000-1, VolpiAG, CH 8952 Schlieren, Switzland), and optionally a camera (e.g.,Olympus ColorView 12), may be used for evaluation of the swollen,stained SAP particles.

As comparative samples, non surface cross-linked SAP particles coatedwith cationic polymers as well surface cross-linked SAP particles coatedwith cationic polymers are submitted to the staining method.

Assessment: Surface Cross-Linked Versus Non Surface Cross-Linked SAPParticles:

Swollen and stained non surface cross-linked SAP particles coated withcationic polymers display a uniform, homogeneous staining throughout theindividual gel particles and essentially plain surfaces.

Swollen and stained surface cross-linked SAP particles coated withcationic polymers on the other hand show characteristic surface patternssuch as flaky surfaces or broken shells. Such pattern cannot be seen insamples of non surface cross-linked SAP particles.

Methods for Making SAP

The basic SAP can be formed in any conventional manner known in the artas discussed above. Typical and preferred processes for producing thesepolymers are described in a long list of literature including manypatent application documents and in particular the textbook “ModernSuper Absorbent Technology” by F. L. Buchholz and A. T. Graham, supra,U.S. Reissue Pat. No. 32,649, U.S. Pat. No. 4,666,983, U.S. Pat. No.4,625,001 U.S. Pat. No. 4,340,706, U.S. Pat. No. 4,506,052 U.S. Pat. No.4,735,987 U.S. Pat. No. 4,541,871, PCT application WO92/16565, PCTapplication WO90/08789, PCT application WO93/05080; U.S. Pat. No.4,824,901; U.S. Pat. No. 4,789,861, U.S. Pat. No. 4,587,30, U.S. Pat.No. 4,734,478; U.S. Pat. No. 5,164,459; German patent application4,020,780, and published European patent application 509,708.

Preferred methods for forming the basic SAP particles are thoseinvolving aqueous solution polymerization methods. The aqueous reactionmixture of monomers is subjected to polymerization conditions, which aresufficient to produce in the mixture, substantially water-insoluble,slightly network cross-linked polymer. The mass of polymer formed canthen be pulverized or chopped to form individual particles.

Finally the SAP particles are coated (but not covalently bonded) withthe cationic polymers according to the present invention. The cationicpolymer can be applied to the SAP particles by simple spraying of asolution comprising the cationic polymer onto the SAP particles in amixer. Alternatively, a solution containing the cationic polymer can bemixed with SAP particles. Preferably, the cationic polymer is providedin an aqueous solution. Alternatively, the cationic polymer is providedin an organic solution.

Other compounds usual in the art, such as salts for pH buffering orneutralization and dust reducing compounds, or other reaction andprocess aids can be used in the conventional manner.

When applying the cationic polymer (preferably the N-polymer) to the SAPparticles it is important not to bind the coating material covalently tothe surface of the SAP particles. It has surprisingly been found, thatupon application of the preferred N-polymers according to the presentinvention, it is neither necessary nor advantageous to bind the cationicpolymer to the superabsorbent particles. To ensure this, the temperatureof the mixing step are critical characteristics to obtain a coatingwithout bonding, which is sufficiently strong on one hand but effectiveenough to allow maintaining the wet integrity of absorbent cores madewith this SAP. The process step of coating the SAP particles with thecationic polymer is preferably carried out at temperatures below 120°C., more preferably at temperatures below 100° C. A test method todetermine, if the cationic polymers are covalently bound to the SAPparticles or not is described in U.S. Pat. No. 6,011,196, columns 17 and18. This test method may be applied on the SAP particles of the presentinvention.

It is even possible to introduce the cationic polymer into absorbentstructures already comprising uncoated SAP particles.

Furthermore, according to the present invention, the cationic polymersare preferably not substantially cross-linked to each other during orafter they have been applied on the SAP particles. Therefore, thecoating on the SAP particles preferably does not comprise cationicpolymers, which are covalently bonded to each other but comprisesindividual cationic polymers. Hence, the cationic polymers coating theSAP particles according to the present invention are preferably neithercovalently bonded to the SAP particle nor are they covalently bonded toeach other.

Absorbent Cores Containing SAP Particles

According to the present invention absorbent cores for disposableabsorbent articles comprise the previously described SAP particleshaving a coating of cationic polymers, with or without other optionalcomponents such as fibers, thermoplastic material, foams, scrims etc.The principle function of such cores is to absorb the discharged bodyfluid, and then retain such fluid, even when subjected to pressures andtensions and torsions normally encountered as a result of the wearer'smovements of absorbent articles made therewith.

In general, the absorbent cores according to the present inventioncontain one or more zones. Generally, a high concentration of SAPparticles, in accordance with the present invention, is desirable toreduce the level of other components, in particular fibrous componentsin order to provide relatively thin absorbent articles.

In measuring the concentration of SAP particles in a given zone of anabsorbent core, the percent by weight of the SAP particles relative tothe combined weight of SAP and any other components (e.g., fibers,thermoplastic material, etc.) that are present in the same zonecontaining the polymer is used.

The fluid storage zone of the absorbent core may comprise other SAPparticles (alone or in addition to the coated SAP particles of thepresent invention), which do not satisfy the above physical criteria ofthe SAP particles of the present invention.

The absorbent core or parts thereof (e.g., the fluid acquisition zone orthe fluid storage zone or parts of these zones) may in addition be fullyor partially wrapped in a tissue, nonwoven or any other suitablesubstrate in order to unitize the assembly. In one preferred embodimentthe core wrap material comprises a top layer and a bottom layer, whichlayers may be sealed together along their edges, e.g., by adhesive. Thetop layer and the bottom layer can be provided from a non-wovenmaterial. The top layer and the bottom layer may be provided from two ormore separate sheets of materials or they may be alternatively providedfrom a unitary sheet of material. Such a unitary sheet of material maybe wrapped around the storage layer, e.g., in a C-fold.

Fluid Acquisition Zone

A key component of diaper of the present invention is the fluidacquisition zone, which comprises the SAP particles of the presentinvention. This fluid acquisition zone serves to quickly collect andtemporarily hold discharged body fluid. A portion of discharged fluidmay, depending upon the wearer's position, permeate the acquisition zoneand be absorbed by the fluid storage zone in the area proximate to thedischarge. However, since fluid is frequently discharged in gushes, thefluid storage zone in such area may not absorb the fluid as quickly asit is discharged. Therefore, the fluid acquisition zone hereof alsofacilitates transport of the fluid from the point of initial fluidcontact to other parts of the absorbent core. In the context of thepresent invention, it should be noted that the term “fluid” includes,but is not limited to, liquids, urine, menses, perspiration, and waterbased body fluids.

The fluid handling function of the fluid acquisition zone is ofparticular importance. The fluid acquisition zone must have sufficienttemporary capacity to rapidly absorb a “gush” of a bodily fluid, andsufficient fluid retention to control the acquired fluid under theinfluence of gravity yet not exhibit excessive fluid retention so as tomake it difficult for fluid storage zone to desorb the fluid acquisitionzone.

In order to acquire, temporarily hold and transport fluids, the swollenSAP particles have to be especially useful for providing interstices toenable the flow of liquid between the swollen SAP particles. This is incontrast to the common use of most prior art SAP particles which weremainly designed for use in the fluid storage zone and therefore mainlyhad to be able to absorb and store high amounts of liquid.

The fluid acquisition zone may be comprised of several differentmaterials including but not limited to: a) nonwoven or woven assembliesof synthetic fibers including polyester, polypropylene, or polyethylene,natural fibers including cotton or cellulose, blends of such fibers, orany equivalent materials or combinations of materials and b) compressedregenerated cellulosic sponges. Particularly preferred materials for thefluid acquisition zone are the SAP particles of the present invention.By applying the SAP particles of the present invention, the fluidacquisition remains desirably thin until it is exposed to an aqueousliquid at which time it rapidly expands so as to absorb the liquid.

The fluid acquisition zone is preferably positioned such that it is influid communication with topsheet, and serves to quickly acquire andpartition body exudates from the wearer's body. Bonding of the fluidacquisition zone the topsheet may enhance the fluid communication byproviding interfacial bonding and preventing topsheet separation formimpeding fluid flow.

A suitable absorbent core comprises an assembly having (a) at least onefluid acquisition zone comprising relatively high concentrations of SAPparticles according to the present invention, preferably at least 50% byweight of the fluid acquisition zone.

In one embodiment, the fluid acquisition zone comprises only one layer.In this embodiment, this layer preferably comprises SAP particles of thepresent invention in a concentration of at least 50% by weight of thislayer, more preferably at least 60% by weight.

In another embodiment, the fluid acquisition zone comprises at least twolayers. In this embodiment, at least one of the layers comprises SAPparticles of the present invention in a concentration of at least 70% byweight of this at least one layer, more preferably at least 80% byweight.

In a preferred embodiment of the present invention, the fluidacquisition zone comprises an upper acquisition layer facing towards thewearer and a lower acquisition layer. In one preferred embodiment theupper acquisition layer comprises a nonwoven fabric whereas the loweracquisition layer preferably comprises the SAP particles of the presentinvention. The acquisition layer preferably is in direct or indirectcontact with the storage layer.

Preferably, the lower layer comprises SAP particles of the presentinvention in a concentration of at least 70% by weight of this lowerlayer, more preferably at least 80% by weight and still more preferablyat least 90% by weight.

Absorbent Articles

FIG. 1 is a plan view of a diaper 20 as a preferred embodiment of anabsorbent article according to the present invention. The diaper isshown in its flat out, uncontracted state (i.e., without elastic inducedcontraction). Portions of the structure are cut away to more clearlyshow the underlying structure of the diaper 20. The portion of thediaper 20 that contacts a wearer is facing the viewer. The chassis 22 ofthe diaper 20 in FIG. 1 comprises the main body of the diaper 20. Thechassis 22 comprises an outer covering including a liquid pervioustopsheet 24 and/or a liquid impervious backsheet 26. The chassis 22 mayalso include most or all of the absorbent core 28 encased between thetopsheet 24 and the backsheet 26. The chassis 22 preferably furtherincludes side panels 30, leg cuffs 32 with elastic members 33 and awaist feature 34. The leg cuffs 32 and the waist feature 34 typicallycomprise elastic members. One end portion of the diaper is configured asthe front waist region 36 of the diaper 20. The opposite end portion isconfigured as the rear waist region 38 of the diaper 20. Theintermediate portion of the diaper is configured as the crotch region37, which extends longitudinally between the front and rear waistregions. The crotch region 37 is that portion of the diaper 20 which,when the diaper is worn, is generally positioned between the wearer'slegs.

The waist regions 36 and 38 may include a fastening system comprisingfastening members 40 preferably attached to the rear waist region 38 anda landing zone 42 attached to the front waist region 36.

The diaper 20 has a longitudinal axis 100 and a transverse axis 110. Theperiphery of the diaper 20 is defined by the outer edges of the diaper20 in which the longitudinal edges 44 run generally parallel to thelongitudinal axis 100 of the diaper 20 and the end edges 46 rungenerally parallel to the transverse axis 110 of the diaper 20.

The diaper may also include other features as are known in the artincluding front and rear ear panels, waist cap features, elastics andthe like to provide better fit, containment and aestheticcharacteristics.

The absorbent core 28 may comprise any absorbent material that isgenerally compressible, conformable, non-irritating to the wearer'sskin, and capable of absorbing and retaining liquids such as urine andother certain body exudates. The absorbent core 28 may comprise a widevariety of liquid-absorbent materials commonly used in disposablediapers and other absorbent articles such as comminuted wood pulp, whichis generally referred to as air felt. Examples of other suitableabsorbent materials include creped cellulose wadding; melt blownpolymers, including co-form; chemically stiffened, modified orcross-linked cellulosic fibers; tissue, including tissue wraps andtissue laminates, absorbent foams, absorbent sponges, absorbent gellingmaterials, or any other known absorbent material or combinations ofmaterials. The absorbent core may further comprise minor amounts(typically less than 10%) of non-liquid absorbent materials, such asadhesives, waxes, oils and the like. Furthermore, the SAP particles ofthe present invention can be applied as absorbent materials.

The absorbent cores of the present invention preferably compriserelatively high concentrations of SAP particles. Consequently, therewill be little or even no fibrous matrix comprised by the absorbentcore, which is able to trap the SAP particles within the fibrous matrixand thus keeping the SAP particles inside the absorbent core. Therefore,the topsheet and the backsheet are preferably joined together (e.g., byadhesive, sewing, needle punching, ultrasonic bonding, by theapplication of heat and/or pressure or any other method known in theart), at least in those regions, where the SAP particles are most likelyto leak out of the absorbent assembly. The respective regions may varydepending on the chosen configuration of the absorbent article.

FIG. 2 shows a cross-sectional view of FIG. 1 taken in the transverseaxis 110. In FIG. 2 illustrates a preferred embodiment of the differentzones comprised by the absorbent cores. In FIG. 2, the fluid acquisitionzone 50 comprises an upper acquisition layer 52 and a lower acquisitionlayer 54, while the fluid storage zone underneath the fluid acquisitionzone comprises a storage layer 60, which is wrapped by an upper corewrap layer 56 and a lower core wrap layer 58.

EXAMPLES Examples of SAP According to the Present Invention

1. Preparation of a Base Superabsorbent Polymer

To 300 g of glacial acrylic acid (AA), 12.837 g ofMethyleneBisAcrylAmide (MBAA) is added and allowed to dissolve atambient temperature. A 2500 ml resin kettle (equipped with a four-neckedglass cover closed with septa, suited for the introduction of athermometer, syringe needles, and optionally a mechanical stirrer) ischarged with this acrylic acid/crosslinker solution. A magnetic stirrer,capable of mixing the whole content, is added. 1166.8 g water are thenadded (the concentration of AA is 20 w/w-%). Most of the water is addedto the resin kettle, and the mixture is stirred until the monomer andwater are well mixed. 300 mg of the initiator (“V50” from WacoChemicals) are dissolved in 20 ml of deionized water. Then, theinitiator solution is added together with any remaining water. The resinkettle is closed, and a pressure relief is provided e.g., by puncturingtwo syringe needles through the septa. The solution is then spargedvigorously with argon via an 80 cm injection needle while stirring atapprox. 300 RPM. Stirring is discontinued after approx. 8 minutes, whileargon spurging is continued. The solution typically starts to gel after12-20 minutes total. At this point, persistent bubbles form on thesurface of the gel, and the argon injection needle is raised above thesurface of the gel. Purging with argon is continued at a lowered flowrate. The temperature is monitored, typically it rises from 20° C. to60-70° C. within an hour. Once the temperature drops below 60° C., thekettle is transferred into a circulation oven and kept at 60° C. for15-18 hours.

After this time, the resin kettle is allowed to cool, and the resultinggel is removed into a flat glass dish. The gel is then broken or cutwith scissors into small pieces (for example in pieces smaller than 2 mmmax. dimension), and transferred into a 6 l glass beaker. 83.26 g of 50%NaOH (the amount of NaOH needed to neutralize 75% of the acid groups ofthe polymer) is diluted with DI water to 2.5 l, and added quickly to thegel. The gel is stirred until all the liquid is absorbed; then, it iscovered and transferred into a 60° C. oven and let equilibrate for 2days.

After this time, the gel is allowed to cool, then divided up into 2 flatglass dishes, and transferred into a vacuum oven, where it is dried at100° C./max. vacuum. Once the gel has reached a constant weight (usually3 days), it is ground using a mechanical mill (e.g., IKA mill) andsieved to obtain SAP particles of the required particle size, e.g., 150to 850 μm.

The amount of MBAA may be adjusted, depending on what properties arerequired from the resulting polymers, e.g., when 1.0 mol % (per mol AA)MBAA is used, the resulting SAP particles have a CCRC of about 20 g/g;when 2.0 mol % (per mol AA) MBAA is used, the resulting SAP particleshave a CCRC of about 16 g/g; when 5.0 mol % (per mol AA) MBAA is used,the resulting SAP particles have a CCRC of about 8 g/g.

If not otherwise stated, all compounds were obtained from AldrichChemicals, Milwaukee, Wis., USA.

2. Surface Cross-Linking Process Step: (Examples I and L Only)

Surface cross-linking of SAP particles is carried out prior to coating.A 150 ml glass beaker is equipped with a mechanical stirrer with aplastic blade, and charged with 100 g of dry SAP particles. Themechanical stirrer is selected in such a way that a good fluidization ofthe polymers can be obtained at 300-500 RPM. A syringe is charged with a4% solution (w/w) of EthyleneGlycolDiGlycidylEther (EGDGE) in1,2-propanediol; another 300 μl syringe is charged with deionised water.

The SAP particles are fluidized in the beaker at approx. 300 RPM, andthe surface cross-linking agent (e.g., 100 μl) is added within 30seconds. Mixing is continued for a total of three minutes. Whilestirring is continued, 7500 μl of water are then added within 3-5seconds, and stirring is continued at 300-500 RPM for another 3 minutes.After this time, the beaker is covered with aluminum foil, and letequilibrate for 1 hour. Then the beaker is transferred to a 140° C.oven, and kept at this temperature for 120 minutes. After this time, thematerial is allowed to cool down, the contents is removed, and thesurface cross-linked SAP particles are obtained. Any agglomerates may becarefully broken by gentle mechanical action. The resulting surfacecross-linked SAP particles may then be sieved to the desired particlesize, e.g., 150-850 μm.

3. Non-Covalently Bonded Surface Coating with a Cationic Polymers

Examples B to D, F to H and K

A 1 l plastic beaker is charged with 50 g of dry SAP particles. Thepartially hydrolyzed cationic polymer of the hydrolysation levels of 30%(for example B), 50% (for examples C, F, G, H and K) and >90% (forexample D), respectively, are added (respectively, Lupamin 9030, Luredur8097, Lupamin 9095 from BASF) at 0.5 w % (e.g., 2.5 g of coatingsolution with 10% cationic polymer content) over about 2 minutes whilemixing by a conventional cooking mixer at ambient temperature. Theadding amount of the cationic polymer are adjusted accordingly to meetdifferent add on levels of 0.1 w % (for example F), 1.0 w % (for exampleG) and 5.0 w % (for examples B, C, D, H and K), respectively, based onthe cationic polymer content of the coating solution. The resultingmaterial is transferred into a glass petri-dish and sticked particlesare disassembled. The coated SAP particles are further dried in the ovenat 60° C. over night (about 16 hours). Any large agglomerates may becarefully broken. The resulting coated SAP particles may then be sievedto the desired particle size, e.g., 150-850 μm.

Example E

A 1 l plastic beaker is charged with 40 g of dry SAP particles. PEI(polyethyleneimine, MW 750,000 from Aldrich Chemicals, Milwaukee, Wis.,USA) is added at 0.4 w % (e.g., 0.78 g of coating solution with 50%cationic polymer content) over about 2 minutes while mixing by aconventional cooking mixer at ambient temperature. The resultingmaterial is transferred into a glass petri-dish and sticked particlesare disassembled. The coated SAP particles are further dried in the ovenat 60° C. over night (about 16 hours). Any large agglomerates may becarefully broken. The resulting coated SAP particles may then be sievedto the desired particle size, e.g., 150-850 μm.

TABLE 1 Samples SAP particle size of all samples: 150 μm-850 um Cross-Add-on linking level of Sample level coating CCRC No: SAP particles [mol%] [w %] [g/g] A Base polymer without coating and 2.0 none 17.0 withoutsurface cross-linking (comparative Example) B Base polymer withPVFA/PVAm 2.0 0.5 16.5 coating 30 mol % hydrolysation degree withoutsurface cross-linking C Base polymer with PVFA/PVAm 2.0 0.5 16.4 coating50 mol % hydrolysation degree without surface cross-linking D Basepolymer with PVFA/PVAm 2.0 0.5 15.9 coating >90 mol % hydrolysationdegree without surface cross-linking E Base polymer with PEI coating 2.01.0 16.8 without surface cross-linking F Base polymer with PVFA/PVAm 2.00.1 16.8 coating 50 mol % hydrolysation degree without surfacecross-linking G Base polymer with PVFA/PVAm 2.0 1.0 16.2 coating 50 mol% hydrolysation degree without surface cross-linking H Base polymer withPVFA/PVAm 2.0 5.0 16.4 coating 50 mol % hydrolysation degree withoutsurface cross-linking I Base polymer without coating; 2.0 none 15.5 withsurface cross-linking (comparative Example) J Base polymer withoutcoating and 3.8 none 10.1 without surface cross-linking (comparativeExample) K Base polymer with PVFA/PVAm 3.8 0.5 10.0 coating 50 mol %hydrolysation degree without surface cross-linking L Base polymerwithout coating; 3.8 none 9.4 with surface cross-linking (comparativeExample)PVFA (=N-vinyl-formamide)/PVAm (=polyvinyl-amine):

-   -   Commercial Name: Lupamin and Luredur, respectively, from BASF    -   Lupamin 9030 was used for the sample with 30% hydrolysation        level (Sample B)    -   Lupamin 9095 was used for the sample with >90% hydrolysation        level (Sample D)    -   Luredur 8097 was used for the samples with 50% hydrolysation        level (Samples C, F to H and K)    -   Mw 400.000    -   Preparation with 10% cationic polymer in solution        PEI (Polyethyleneimine):    -   Used for sample E)    -   Mw 750.000    -   Preparation with 50% cationic polymer in solution        All comparative examples are examples of SAP particles, which        are not coated with cationic polymers. These comparative        examples are not within the scope of the present invention.

TABLE 2 Effects of Coating on Permeability Permeability Sample No:Coating [10⁻⁷ cm³sec/g] A Base polymer without coating 65 B Base polymerwith PVFA/PVAm coating 198 30 mol % hydrolysation degree C Base polymerwith PVFA/PVAm coating 1012 50 mol % hydrolysation degree D Base polymerwith PVFA/PVAm coating 268 >90 mol % hydrolysation degree E Base polymerwith PEI coating 339

TABLE 3 Effects of add-on level on permeability Add-on level of coatingPermeability Sample No: [w %] [10⁻⁷ cm³sec/g] A None 65 F 0.1 186 C 0.51012 G 1.0 1086 H 5.0 405

TABLE 4 Effects of Coating on Wet Porosity Add-on level of Samplecoating Wet No: Coating [w %] Porosity A Base polymer without coatingnone 0.206 B Base polymer with PVFA/PVAm coating 0.5 0.285 30 mol %hydrolysation degree C Base polymer with PVFA/PVAm coating 0.5 0.339 50mol % hydrolysation degree D Base polymer with PVFA/PVAm coating 0.50.306 >90 mol % hydrolysation degree E Base polymer with PEI coating 1.00.297 F Base polymer with PVFA/PVAm coating 0.1 0.247 50 mol %hydrolysation degree G Base polymer with PVFA/PVAm coating 1.0 0.318 50mol % hydrolysation degree H Base polymer with PVFA/PVAm coating 5.00.245 50 mol % hydrolysation degree

TABLE 5 Effects of surface cross-linking versus cationic polymer coatingon permeability Sample Cross-linking Surface-cross- permeability No:level [mol %] linking Coating [10⁻⁷ cm³sec/g] A 2.0 no No 65 C 2.0 noYes 1012 I 2.0 yes no 306 J 3.8 no no 109 K 3.8 no yes 1426 L 3.8 yes no263

All documents cited in the Detailed Description of the Invention are, inrelevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. An absorbent article comprising a substantially liquid pervioustopsheet, a substantially liquid impervious backsheet and an absorbentcore between said topsheet and said backsheet, wherein said topsheet andsaid backsheet are at least partially joined together, and wherein saidabsorbent core comprises a storage zone and a fluid acquisition zonediscrete from and adjacent to said storage zone, wherein said fluidacquisition zone comprises surface-coated superabsorbent polymerparticles, wherein: a) said surface-coated superabsorbent polymerparticles comprise superabsorbent polymer particles having a surface,and cationic polymers on the surface of the superabsorbent polymerparticles, wherein said cationic polymers have 1 to 25 mol/kg, referringto the total weight of the cationic polymers, of cationic groups, whichcan be protonated, wherein said cationic polymers are not substantiallycovalently bound to said superabsorbent polymer particles; and b) saidsurface-coated superabsorbent polymer particles of the fluid acquisitionzone have a Cylinder Centrifuge Retention Capacity (CCRC) of less than20 g/g; further wherein the storage zone does not comprisesuperabsorbent polymer particles which have a CCRC of less than 25 g/g;wherein said fluid acquisition zone comprises said surface-coatedsuperabsorbent polymer particles in a concentration of at least 50% byweight of said fluid acquisition zone; and said superabsorbent polymerparticles of the fluid acquisition zone are substantially free ofcovalent surface cross-links.
 2. Absorbent article according to claim 1,wherein said fluid acquisition zone comprises at least one layer andwherein said surface-coated superabsorbent particles are comprised bysaid at least one layer in a concentration of at least 70% by weight ofsaid acquisition layer.
 3. Absorbent article according to claim 1,wherein said superabsorbent polymer particles of the fluid acquisitionzone are slightly network cross-linked to each other.
 4. Absorbentarticle according to claim 1, wherein said cationic polymers arenitrogen-containing cationic polymers.
 5. Absorbent article according toclaim 4, wherein said nitrogen-containing cationic polymers arepartially hydrolyzed.
 6. Absorbent article according to claim 5, whereinsaid nitrogen-containing cationic polymers are hydrolyzed in the rangeof 30% to 80%.
 7. Absorbent article according to claim 1, wherein saidcationic polymers are primary amines, secondary amines or tertiaryamines.
 8. Absorbent article according to claim 1, wherein saidsurface-coated superabsorbent polymer particles of the fluid acquisitionzone have a permeability of at least 400×10⁻⁷ cm³ sec/g.
 9. Absorbentarticle according to claim 1, wherein said surface-coated superabsorbentpolymer particles of the fluid acquisition zone have a Wet Porosity ofat least 0.20.